The present invention relates to burning method and apparatus for burning gaseous fuel, and more particularly, it relates to a low NOx burning method and a low NOx burner apparatus suitable to reduce the generation of nitrogen oxide (referred to as "NOx" hereinafter) during the burning of the fuel.
The NOx generated during the burning of the fuel includes thermal NOx formed by oxidizing the nitrogen in the combustion air, and fuel NOx formed by oxidizing the nitrogen in the fuel.
In general, most of the gaseous fuels used in the industry, such as natural gas, have a low nitrogen content, except special fuels such as fuel manufactured by gasifying the coal. The NOx generated by burning the fuel and having the low nitrogen content is the thermal NOx formed by oxidizing the nitrogen in the air. The formation of the thermal NOx greatly depends upon the temperature, and an amount of the generated NOx increases in proportion to the flame temperature. Conventional low NOx burning techniques are mainly based on the principle of reduction of the flame temperature, and comprise an exhaust gas recycling method wherein combustion exhaust gas having low density of oxygen is mixed into the combustion air or a two-stage burning method in which the combustion air required for perfect combustion is introduced after the fuel is burned with a low air ratio, without burning the fuel with the air ratio of 1 or thereabout (which results in the maximum flame temperature), i.e., without burning the fuel with a theoretical air amount or thereabout required for the perfect combustion, or the like. Further, there exists a so-called furnace denitrification technique which does not have the purpose of reducing the flame temperature and wherein the fuel is introduced at two stages and a part of the fuel is used as a reducing agent for the generated NOx. It was already ascertained that these burning technique were effective to suppress the generation of the NOx by using them with the actual boiler and the like. However, with these conventional burning techniques, there arose problems that the burning flame was lengthened, and the burner was large-sized and expensive. Accordingly, an economical burning technique has been still requested.
In order to make the burning apparatus compact, i.e., to increase the load for the burning apparatus so that a large amount of fuel can be burned by the small-sized burning apparatus, the length of the flame must be shortened.
In the normal burning apparatus, a so-called diffusive flame technique wherein the fuel and the combustion air are discharged from different nozzles and are mixed together in the burning apparatus is often employed, for the reasons that it can prevent a so-called back fire, in which phenomenon the flame returns to the fuel supplying tube and/or air supplying tube, since the nozzle for the fuel is different from the nozzle for the air, thereby obtaining the safety operation of the burning apparatus, and that a stable flame can be obtained by creating a low flow speed area in the mixing portion between the fuel and air. However, with diffusive flame, the burning time is increased by the time required for mixing the fuel and air and the flame tends to be lengthened.
For this reason, in order to shorten the flame length, a premixing flame technique wherein the premixed fuel and air mixture is introduced into the burning apparatus is employed. However, the premixing flame technique has a severe condition for forming the stable flame and limits the supplying speed and the like. Particularly, a condition that the flame is stably formed is obtained when the fuel is burned with the air ratio (ratio between the amount of the supplied air and the theoretical amount of air required for perfectly burning the fuel) of 1 or thereabout. That is to say, this condition is the same as a condition that the flame temperature is highest. Thus, in this condition, the thermal NOx is readily generated. For this reason, a burning method wherein the flame burned with the air ratio less than 1 and the flame burned with the air ratio more than 1 are simultaneously formed, thus burning the fuel without using the air ratio leading to the increased temperature of the flame has been used. For example, Japanese Patent Publication No. 62-62245 discloses a premixing combustion burner comprising a premixing flame type nozzle for discharging combustion air having an amount more than a theoretical air amount required for burning liquid fuel into a combustion chamber, a diffusive flame type nozzle for discharging combustion air having an amount less than the theoretical air amount required for burning the liquid fuel, and a fuel spray for spraying the liquid fuel into the air flow discharged from the nozzles, and wherein a recycling exhaust gas discharging nozzle for discharging the recycling exhaust gas in parallel with air discharged from the diffusive flame type nozzle is arranged between the premixing flame type nozzle and the diffusive flame type nozzle and a direction of the air discharged from the premixing flame type nozzle is inclined so that the premixing flame type nozzle is diverged from the diffusive flame type nozzle.
Also in this case, the area where the flame temperature is increased can be avoided and it is expected that the generation of the NOx is greatly reduced; however, since the flame burned with the low air ratio is the diffusive flame, an area having the air ratio of 1 or thereabout is created in the mixing area between the fuel and air, thus creating an area where a large amount of NOx is generated. Further, since the flame burned with the low air ratio and the flame burned with the high air ratio are separately formed, there arises a problem that the flame is lengthened by the time when the both flames are mixed.
Further, a burner using a pilot flame for holding unstable premixing flame is already known, as disclosed in Japanese Patent Unexamined Publication No. 62-210313. However, in this conventional burner, since the main flame has the same air ratio as that of the pilot flame, an operation range where the premixing flame is stably formed is very narrow.
In addition, Japanese Patent Unexamined Publication No. 61-41810 teaches a burning method wherein the generation of the NOx is reduced by mixing the flame burned with the low air ratio and the flame burned with the high air ratio.